Magnetic materials



Patented Aug. 14, 1945 UNITED STATES PATENT OFFICE Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York No Drawing. Application October 23, 1941, Serial No. 416,217

Claims.

This invention relates to permanent magnet alloys and to magnets produced therefrom which are caused to have more desirable properties as permanent magnets by a cold working treatment. The invention also relates to and includes methods of treating and producing such improved alloys and permanent magnets.

An object of the invention is the production of better and more eflicient permanent magnet materials.

In my copending application Serial No. 311,735, filed December 30, 1939, of which this application is a continuation-in-part certain phases of the present invention were disclosed or claimed or both disclosed and claimed but were divided therefrom because of requirements for division, requirements for election, or alleged indefiniteness of generic claims; nevertheless the benefit of the filing date of said application is claimed for the present application to whatever extent and under whatever provisions of law may be applicable.

A feature of the present invention is the discovery that permanent magnet compositions, such as those described hereinafter, may be improved by reduction of the cross section thereof by a stage of cold working during their preparation and prior to the final heat treatment. Such working is preferably accomplished by methods which produce elongation and prevent lateral spreading. Methods that have been found beneficial include swaging, rolling with grooved rolls and wire drawing. It appears that the result of improved permanent magnet properties is manifested chiefly in the direction of elongation.

' Reduction of thickness by such rolling of a metal sheet as permits the material to spread laterally is only partially effective.

The evidence is that materials of or produced according to the present invention generally are anisotropic and their improved properties are manifested chiefly in the direction in which the elongation takes place.

In the said copending application it is pointed out that there are classes of materials which, if maintained at a certain high temperature but below the melting point, assume a form known as the gamma" phase, and if the material is thereafter cooled to around room temperature, it tends to assume the alpha phase and thereafter if maintained at around 500 to 800, a finely dispersed small amount of the gamma" phase forms in the alpha phase. This brief statement is to be read in the light of the information, diagrams and discussion of said copending application.

As pointed out in the said copending application, now Patent No. 2,298,225, dated October 6, 1942, a suitable procedure for preparing permanent magnet material is to melt the material and cool it to room temperature to produce formation of the alpha phase and thereafter elevate the material to a higher temperature between 500" C. to 800 C. for a length of time such as is necessary to allow a small amount of the gamma phase to precipitate in a highly dispersed form in the alpha" phase. This produces dispersion hardening in the material and produces a very effective permanent magnet material. This method of forming permanent magnet materials is different and seemingly involves a difierent principle than that heretofore employed with respect to many others, such as alloys of iron, cobalt and molybdenum, in that, in the present instance the material is converted into a low temperature alpha" phase and thereafter has a small amount of the high tempera ture gamma phase precipitated therein in a fine and dispersed state. In the usual case of permanent magnets hardened by precipitation the high temperature phase is preserved and a small amount of the low temperature phase is precipitated.

In some alloys of the type under discussion the gamma to alpha" transformation tends to occur at low temperatures at which the transformation becomes sluggish and non-equilibrium conditions exist.

Whether this is the case or not cold rolling these alloys in grooved rolls or working them mechanically by any equivalent method causes or expedites the formation at room temperature of the alpha" phase and brings them into equilibrium. Some alloys of this type may not change completely to the alpha phase at roozii temperature unless given the above treatment. By the rolling treatment others change to alpha" phase only in part. Whether complete or partial this result is one beneficial aspect of the present invention. However, there is a second beneficial aspect which is due to crystal orientation. -X-ray measurements confirm this and magnetic tests show the best permanent magnet properties in the direction of elongation. Either or both of these beneficial aspects may be present in some degree.

After the alloys are brought into the "alpha? phase by the combination of cooling and cold working they are raised to a temperature in the range of temperatures which causes the precipitation in finely dispersed particles of some of the gamma" phase. This results in the production of magnetic material having effective and desirable properties as permanent magnetsin the direction of elongation. In every case care must be usednottoraisethematerialtotoohighatemperature in order not to lose the effect of cold working,

A frequently used criterion of the desirability of permanent magnet materials is the product of coercive force" and "residual induction." A more accurate figure of merit is that of "maximum energy product," which on the demagnetization portion of the hysteresis loop, is the product of induction B and magnetizing force It at a point where this product is the greatest. See Wahl, Applied Magnetism," pages 42 to 45, inclusive.

The above criterion is independent of the matter of cost, ease or diiiiculty of preparation miling or drawing, brittleness, availability of raw materials and other factors which must receive consideration for many practical purposes and applications. Thus a permament magnet composition which has an absolute maximum energy product less than some other which is less available or suitable from some one or more of the above aspects may be a valuable contribution to the art, if, for example, it may be made to possess a considerably greater maximum energy product.

Among objects of the present invention are to provide better permanent magnets at little or no increase in cost; to produce useful permanent magnets from materials not hitherto known to possess permanent magnet properties to a useful extent; to improve the properties of compositions known to have useful permanent magnet properties; to produce useful permanent magnets from cheap or readily available raw materials, to increase either the product of "coercive force" and residual induction or the maximum energy product of magnetic materials, to provide elongated tapes or strips of magnetic material having useful magnetic energy products" and to provide useful magnets from materials which may be worked with facility.

The subject-matter of the invention or discovery comprises a range or group of magnetic alloys which by appropriate combined heat treatment and cold rolling or elongating treatment will have one or the other or both of the abovenamed magnetic products or the coercive force or the residual induction increased in the direction of elongation by from one hundred to sev eral hundred per cent as compared to the material in the cast or unsuitably heat treated or unsuitably worked condition.

A feature of the invention or discovery is the lack of any necessity for quenching the material at any time in order to improve its properties; however, quenching or rapid cooling upon casting may be practiced. One of the beneficial aspects of the discovery is that the cooling rate may vary widely as convenience or necessity demantis.

The alloys may be prepared in the form of rods, bars, wire or tapes. A suitable treatment for any specimen is first to give it the desired amount of cold working plus a low temperature bake. No other heat treatment is necessary. Satisfactory results have been obtained with reduction in area of 75 per cent, although this exact amount is by no means critical.

In greater detail, the material is cast, which gives it the necessary high temperature treatment; it may be given a further or special heat treatment at around 800 to 1300 C. The cooled material, which may be cooled to room temperature slowly or rapidly, and must be cooled substantially below about 600 C., is forcibly elongated by rolling with grooved rolls, swaging, drawing through dies or by combinations of these methods or by any similar method or methods so as to reduce the cross section in one or several steps over a range from a small amount to a small fraction of the original cross section. The material is then heated to a temperature generally in the region 500 to 800 C. and maintained for a long time at the bottom of the range, one to several hours in the middle and much less if the temperature is at the top of the range. Too high a temperature destroys the effect of the cold rolling and too low a temperature or too short a time is only partially effective. A quenching treatment is unnecessary.

Compositions which respond to this type of treatment are:

Carbon above about 0.1 per cent is not an essential or desirable constituent, other incidental impurities may be present in small amounts. The iron content may vary between and per cent and the nickel and manganese may make up the balance according to the general ranges indicated.

Thus there are described certain magnetic materials, a heat treatment to improve them, a further improvement by a precedent working treatment, and avoidance of necessity for quenching.

These compositions possess or may be made to possess substantial permanent magnet properties and are notably improved in properties by a treatment of the kind described. They may be prepared from raw materials which are ordinarily cheap and readily available. They respond to the described treatment and may be made to acquire magnetic properties increased around per cent or more as a result of the treatment described as compared with identical compositions as cast. In some cases the increase in energy product due to cold rolling alone may be 100 per cent as compared to the same material heat treated but not cold rolled.

The apparatus illustrated in the above-noted copending application or any other suitable apparatus may be used to perform the cold rolling or elongating process.

The finished magnets may be magnetized by any usual method.

What is claimed is:

l. A magnetic composition or alloy composed of iron 75 to 85 per cent, nickel 4 to 10 per cent and manganese 10 to 21 per cent.

2. A composition according to claim 1, which has been forcibly elongated while cold and thereafter heated above 500 C. and not above 800 C.

3. A permanent magnet composed of iron 75 to 85 per cent, nickel 4 to 10 per cent and manganese 10 to 21 per cent.

4. An article having properties adapted for permanent magnet use composed of an alloy comnet which comprises composing an alloy of iron 75 to 85 per calm-nickel 4 to 10 per cent and manganese 10 to 21 per cent, passing it into or through the temperature range 800 C. to 1300 0., cooling the material to the general region of room temperature, forcibly elongating the material while cold, and thereafter maintaining it at 500 C. to 800 C., cooling it, and strongly magnetizing it along the general axis of the direction of elon- ETHAN A. NESBI'I'I.

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